The basic principle of such analysis is referred to as the principle of W. Coulter, which is described in Coulter U.S. Pat. No. 2,656,508. This patent describes an apparatus for counting and classifying microscopic particles suspended in an electrically conducting liquid, such as a saline solution. The electrical conductivity of the liquid is different from that of the particle to be detected and measured. A pair of containers are in fluid communication through a small aperture through which the liquid suspension is caused to flow. The aperture has dimensions which are greater than and are within one to five orders of magnitude of those of the particles. Electrodes are disposed on both sides of the aperture in contact with the liquid and between which a constant current I flows, resulting in a potential difference between these electrodes. The electrical resistance R.sub.1 between the electrodes is affected by the presence and size of the particles in the liquid within the aperture. The change .DELTA.R.sub.1 in electrical resistance results in a change of electrical current flowing through the liquid as the particle moves through the aperture. In the Thevenin equivalent of this structure a voltage pulse is generated whose magnitude may be represented by the equation e.sub.1 =I.DELTA.R.sub.1. Thus there are produced a series of discrete signal voltage pulses as the particles move in sequence through the aperture. The amount of resistance change .DELTA.R.sub.1 has been shown to be substantially proportional to both the aperture resistance R.sub.1 and the volume of the particle.
Examples of particle analyzing apparatus operating on the principle disclosed in Coulter U.S. Pat. No. 2,656,508 appear in U.S. Pat. Nos. 3,380,584; 3,706,030; 3,710,933; 3,793,587; 3,924,180; and 3,944,917, among others.
Because the voltage across the aperture of an apparatus operating on the principle of W. Coulter normally is very large compared to the signal voltage produced by a particle traversing the aperture, it has been customary to use a.c. coupling between the aperture and the input to the preamplifier of the system. D.C. coupling at this point in the system is undesirable because the high d.c. bias (of the order of 50 volts) sometimes required across the aperture would saturate the amplifier and render it useless. The ratio of signal voltage to the bias voltage may be 1 to 10,000. Where the bias voltage is 50 volts, the signal input voltage may be of the order of 5 mv (Milli-volts).
A.C. coupling to the preamplifier creates problems because of negative undershoot below the base line in the output pulse waveform from the preamplifier. The area of the undershoot equals the area of the pulse above the base line in a.c. coupling. A Coulter Counter.RTM. of the type manufactured by Coulter Electronics, Inc. of Hialeah, Fla., for counting particles is often used to count a wide range of particle sizes within a particle sample, for example, from 2% of aperture diameter to 40% of aperture diameter which represents a particle volume range of 8000:1. Since wide pulse widths are characteristic of large Coulter.RTM. apertures above 500 um (microns) diameter and large time constants are required in the aperture coupling circuit for amplifying wide pulses, the undershoot must become negligibly small before the next succeeding pulse appears in the output of the preamplifier in order to avoid a size error in the measurement of the following particle.
An example of wide pulses is given by a 1,000 um (microns) aperture covering a 1 ms (milli-second) time interval. A narrow pulse may result from a 70 um aperture covering a 20 us time interval. Since the negative undershoot of the preamplifier output pulse caused by capacitive coupling might last for several pulse widths, it is essential to reduce the undershoot to an absolute minimum for longer duration pulses in order to achieve a satisfactory count rate; that is, one that is capable of obtaining a reasonable statistical sample of particles within a reasonable time interval and without appreciable size errors.
If an attempt is made to obtain d.c. coupling by applying a counter or bucking d.c. voltage between the aperture and the preamplifier of a magnitude equal to the aperture voltage, then stability or voltage drift problems are created. Such voltage drift problems are caused by temperature changes in the electrolyte (liquid suspension) and to chemical reactions in the sensor, and would become enormous in the output of the entire amplifier chain due to the high gain needed in the Coulter Counter.